Friction stir welding

ABSTRACT

The present invention relates to a method of friction welding for joining of members and more particularly to so-called friction stir welding based on a relative rubbing movement between a probe of harder material and members to be joined. The present invention also is directed to an improved tool to be applied in the friction welding process.

The present invention relates to a method of friction welding forjoining of (metal) members and more particularly to so-called frictionstir welding based on a relative rubbing movement between a probe ofharder material and members to be joined. The present invention also isdirected to an improved tool to be applied in the friction weldingprocess.

Friction welding based on the principle of "rubbing" of articles to bejoined together so as to generate a sufficient amount of heat, thusgenerating plasticised conditions in the adjacent surfaces, has beenknown and practised for several decades. The frictional heat isgenerated solely by the two to be joined components. However, theresulting weld seams suffer from a number of disadvantages inherent inthe applied processes. The main drawback which considerably limits theapplicability of friction welding is that at least one of the componentsto be welded has to be axis-symmetric. Consequently, such process is notapplicable for e.g. structural applications requesting provision oflongitudinal continuous welds.

The improved version of this welding so-called friction stir welding, isknown from W093/10935. A probe (third body) of a narder material thanthe treated workpieces is applied in the welding process. Friction stirwelding is based on a relative cyclic movement between the probe and theworkpieces, urging the probe and workpieces together to create aplasticised region in the workpiece region due to generated frictionalheat, stopping the relative cyclic movement and allowing the plasticisedmaterial to solidify. Thus no heat is generated due to a relative motionbetween the workpieces to be joined. The method is illustrated byseveral examples of different workpiece materials (plastics, metals),applications (reparation of cracks, sealing, joining) and embodiments ofthe applied probe.

Neither the above disclosed method and apparatus, when applied forjoining of extruded shapes into structures for critical applications,can meet the request for high integrity welds free from voids and propermetallurgical bonding of structural parts, nor a demand for provision ofwelds in a lap configuration.

In order to achieve a proper consolidation of the weld metal the probebottom part (shoulder) must maintain during the whole welding operation(forward movement) in an intimate contact with surface of the joinedmembers. If the probe shoulder during this forward movement eventemporarily "lifts" from the surface a small amount of plasticisedwelding material will be expelled behind the probe thus causingoccurrence of voids in the-weld since there is no available material tofill the vacant space after the expelled material.

Furthermore there is another limitation connected to use of "smooth"welding probe known from the prior art, namely low welding speeds arerequired in order to achieve a sufficient frictional heating of thematerial (contact time between the probe and the joined members), and toensure a sufficient flow of the plasticised welding material.

It is therefore an object of the present invention to provide animproved method of friction stir welding, ensuring high integrity weldsfree from voids and exhibiting a smooth quality surface.

A further object of the invention is to improve the present knownfriction stir butt welding method to provide also lap welds and three ormore components joint configuration.

Another object of the present invention is to provide a new type ofprobe ensuring uniform homogenized weld seams exhibiting reduced heataffected zone.

The above and other objects are achieved in accordance with the presentinvention by provision of a method for friction stir welding and anapparatus (probe) as defined in the accompanying claims 1 and 7,respectively.

Other objects, specific features and advantages of the present inventionwill be apparent from the following detailed descriptions of preferredembodiments with reference to the accompanying drawings, FIGS. 1-5,where:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic perspective view of the welding apparatus/processemployable in the present invention,

FIG. 2 illustrates in an enlarged cross-sectional (partial) view theconfiguration of the two components probe,

FIG. 3 shows schematically principal features of the novel friction stirwelding,

FIG. 4 shows graphically the optimal relation between welding androtational speed and applied pressure on the probe, and

FIGS. 5a-e are fragmentary, schematic perspective views of differenttypes of the provided weld seams.

Referring to the drawings, particularly to FIG. 1, a non-consumableprobe 1 comprising a rotational cylindrical body 2 having an upper part22 being connected to a power source, e.g. an engine (not shown in thedrawing), and a bottom part 23 provided with a separate pin 24, isapplied for joining (welding) of two butt to butt arranged members(metal plates) 3 and 4. By insertion of the probe 1 between the members3,4 under pressure and rotation in order to provide adequate frictionalheating a butt seam weld 5 is subsequently provided during atransitional movement along the butt arranged members.

The special inventive configuration of the composite two parts probe 1,the bottom part 23 (shoulder) of the probe and outer shape(configuration) of the applied pin 24 according to the present inventionis more apparent from FIG. 2, where the bottom part 23 of the rotationalcylindrical body 2 exhibits a concave surface, while the pin's 24 outersurface is provided with alternately protruding and recessed parts alongits longitudinal axis.

Even if a conventional screw thread on the outer pin surface isapplicable and will improve the weld quality, in a preferred embodimentof the probe pin the surface configuration, as illustrated in theFigure, comprises advantageously two or more separate blades 25vertically disposed and laterally protruding from the central portion ofthe pin.

Microstructure of welding seams provided by this novel type of the pinexhibits sound joints composed of interlocked plasticised segments ofthe joined members with minimal material turbulence.

The composite design of the applied probe 1 comprising a separate probepin compared to the monolithic design of the previously known probespresents several advantages. Firstly, the tool (probe) is capable ofadjustment of the depth of pin insertion, thus offering a processflexibility. Secondly, the pin portion can be easily interchangedbetween different tool units (shoulder/holder) and thus adapted forwelding of members having various wall thickness.

The innovative features of the new tool and improved process of frictionstir welding will be readily understood from the schematical sketch ofthe probe and welding performance as illustrated in FIG. 3, where Wdenominates the thickness of the welded members, ω welding speed, Rradius of curvature of the concave probe shoulder, F downward force(pressure) applied on the probe, r_(s) probe shoulder radius, and trepresents "undercut" of the shoulder into the welded material.

The concave bottom surface 23 of the probe 1 exhibiting slightinclination in relation normal to the welding surfaces combined with theabove described screw configuration of the pin 24 (not shown in theFigure) ensures that the plasticised material is driven both verticallyand laterally in a weld zone, thereby transforming metal betweendifferent levels across the weld profile. The concave shape of the probeshoulder provides a certain non-symmetric compression on the surface ofthe members resulting in vertical flow/transition of material, This inco-operation with horizontal movement of material due to the pin's outerconfiguration results in high quality void free seams.

FIG. 4 illustrates schematically an optimum relation between a downwardpressure F/πr² _(s) in N/mm² applied on the probe and the actual weldingspeed ω(mm/min) at different rotational speeds ensuring a sound voidfree weld having a smooth surface. The actual values of optimalpressure/welding speed are dependent on several factors, e.g. appliedmaterial of the joined members (Al-alloys), shoulder geometry etc.

Several welding trials conducted with probes of different diameters showthat reduction of the probe shoulder's diameter related to the actualwall thickness of the joined members has a beneficial effect on thequality of the provided joints/seams besides possibility of increasingthe welding speed.

Thus, reduction of the probe shoulder's diameter from 20 mm to 15 mm andfurther to 10 mm applied for joining of 3 mm thick flat extrusions ofalloy 6082.50 allowed an increase of the welding speed from 0.3 m/min to0.8 m/min achieving high quality porefree welds exhibiting reduced HAZ(heat affected zone). This is a combined result of decreased heat inputand its focusing towards the vicinity of the formed seam allowing forincrease of the welding speed and reduced downward force applied on theprobe resulting in distortion free welded structures.

A simple formula defining an optimal relation between the radius of theprobe shoulder (r_(s)) and the wall thickness of the welded members Wwill be:

    5W/2≧r.sub.s ≧W/2

The material of the probe is harder than the workpieces/members to bejoined. Typically for application on aluminium (alloy) members thematerial should exhibit a good strength at elevated temperatures, e.g.hot work steel, high speed steel or cermet material can be applied.

FIGS. 5a-e display schematically in fragmentary perspective viewsdifferent types of welds provided by the method and probe according tothe present invention, where FIG. 5a shows a conventional butt weld,FIG. 5b shows a T-section connection between two members, FIG. 5c is anoverlap weld seam, FIG. 5d shows another variant of T-section composedof three members, and finally FIG. 5e illustrates provision of a cornerweld between two members arranged in a normal plane to each other.

Thus application of the new and improved tool design according to thepresent invention allows for increased welding speeds while ensuringsufficient generation of frictional heat. This effect is achieved due toan extended contact/heating time per volume unit of the welding materialand closer localization of the generated heat along the welding line.Further more substantial forging forces (pressure) are applied on thematerial when leaving the welding probe. Optimal combination of theabove effects results in high quality welds both with regard to themetallurgical and mechanical properties due to a homogenized weldexhibiting no porosity through the whole weld cross-section.

We claim:
 1. A method of friction stir welding of members, particularlyfor joining of extruded assembled shapes, comprising steps of urging andsecuring the assembled members towards each other, entering theassembled members along the joining line by a probe of material harderthan the material of joined members under rotating movement whichgenerates a frictional heat, thereby creating a plasticised region inthe adjacent members' material, the method further comprising ahomogenization of the resulting weld seam ensured by an enhanced flow ofplasticised material both perpendicularly and vertically to thelongitudinal extension of the adjacent assembled members by exposing thecreated plasticised material to a perpendicular pressure along thesurface of the members and causing simultaneous material flow along theprobe pin in the vertical direction allowing the plasticised material tosolidify behind the probe.
 2. Method according to claim 1, characterizedin that the probe bottom part (23) engages the adjacent surfaces of themembers to be joined under a slight inclination from normal to weldingsurfaces.
 3. Method according to claim 1, characterized in that theprobe pin's (24) outer configuration ensures a vertical and lateral flowof the plasticised material across a weld zone.
 4. Method according toclaim 1, characterized in that two or more members are welded providinga T-type welding joint.
 5. Method according to claim 1, characterized inthat the provided welded seam is a lap welding joint.
 6. Methodaccording to claim 1, characterized in that the provided weld seam is acorner weld connecting two members arranged substantially at any angleto each other.
 7. A non-consumable probe (1) for stir friction weldingof members comprising a rotational, substantially cylindrical body (2),having an upper part (22) connected to a power source and a bottom part(23) provided with a pin (24), characterized in that the bottom part(23) of the probe has a concave face and the attached pin (24) is aninterchangeable part of the probe exhibiting a threaded surfaceconfiguration.
 8. Probe according to claim 7, characterized in that thepin is provided with at least a pair of vertically disposed bladeslaterally protruding from the pin's central portion.
 9. Method accordingto claim 2, characterized in that the probe pin's (24) outerconfiguration ensures a vertical and lateral flow of the plasticisedmaterial across a weld zone.
 10. Method according to claim 2,characterized in that two or more members are welded providing a T-typewelding joint.
 11. Method according to claim 3, characterized in thattwo or more members are welded providing a T-type welding joint. 12.Method according to claim 9, characterized in that two or more membersare welded providing a T-type welding joint.
 13. Method according toclaim 2, characterized in that the provided welded seam is a lap weldingjoint.
 14. Method according to claim 3, characterized in that theprovided welded seam is a lap welding joint.
 15. Method according toclaim 9, characterized in that the provided welded seam is a lap weldingjoint.
 16. Method according to claim 2, characterized in that theprovided weld seam is a corner weld connecting two members arrangedsubstantially at any angle to each other.
 17. Method according to claim3, characterized in that the provided weld seam is a corner weldconnecting two members arranged substantially at any angle to eachother.
 18. Method according to claim 9, characterized in that theprovided weld seam is a corner weld connecting two members arrangedsubstantially at any angle to each other.